Inkjet printing mechanisms may be used in a variety of different products, such as plotters, facsimile machines and inkjet printers, to print images using a colorant, referred to generally herein as "ink." These inkjet printing mechanisms use inkjet cartridges, often called "pens," to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms carry an ink cartridge with a full supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as "off-axis" systems, propel only a small ink supply with the printhead carriage across the printzone, and store the main ink supply in a stationary reservoir, which is located "off-axis" from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the printhead cartridge. In multi-color cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color also being referred to herein as a "pen."
Each pen has a printhead formed with very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located side-by-side on the printhead, parallel to one another, and perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The height of this band is known as the "swath height" of the pen, the maximum pattern of ink which can be laid down in a single pass.
It is apparent that the speed of printing a sheet can be increased if the swath height is increased. That is, a printhead with a wider swath would require fewer passes across the sheet to print the entire image, and fewer passes would increase the throughput of the printing mechanism. "Throughput," also known as the pages-per-minute rating, is often one of major considerations that a purchaser analyzes in deciding which printing mechanism to buy. While merely lengthening the nozzle array to increase throughput may seem to the inexperienced an easy thing to accomplish, this has not been the case. For thermal inkjet pens in particular, there are some physical and/or manufacturing constraints to the size of the substrate layer within the printhead. In the past, inkjet printheads have been limited in swath height to around 5.4 mm (millimeters) for tri-chamber color printheads, and around 12.5 mm (about one-half inch) for monochrome printheads, such as black printheads.
To clean and protect the printhead, typically a "service station" mechanism is mounted within the plotter chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit or other mechanism that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station.
After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the face of the printhead. Other service stations include auxiliary wiping members to clean areas of the pen adjacent to the ink ejecting nozzles. For instance, a pair of "mud flaps" in the models 720C and 722C DeskJet.RTM. color inkjet printers wipe regions beside the color nozzles, while a "snout wiper" in the models 2000 and 2500 DesignJet.RTM. color inkjet plotters wipe a rear vertical surface underneath an electrical interconnect region of the pen, with these printers and plotters both being sold by the present assignee, the Hewlett-Packard Company of Palo Alto, Calif.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper, as well as on recently developed specialty coated papers, transparencies, fabric and other media.
Indeed, keeping the nozzle face plate clean for cartridges using pigment based inks has proven quite challenging. In the past, multiple inkjet printheads were wiped simultaneously, all at the same speed, which was fine when all the cartridges contained the same type (albeit different colors) of ink. However, these pigment based inks are less viscous than the dye based inks, so the pigment based inks require a slower wiping speed than that previously needed for dye based inks. Yet, there is a lower limit to the wiping speed because too slow a wipe wicks excessive amounts of ink from the dye based pens. This excess dye based ink eventually builds-up a residue on the wiper, leading to less effective wiping in the future, as well as other problems. For instance, excess residue around the wipers may lead to ink build-up around the service station, which could contaminate the caps. Printhead cap contamination may lead to shorter cartridge life because ineffective capping may induce failures in the printhead.
Actually, a scrubbing type of wiping routine is preferred to clean the tar-like pigment ink residue from the printheads. If a faster wipe was used to accommodate the dye based inks, the wiper for the pigment based ink is prevented from making full contact with the residue. Instead, the wiper skips over bumps formed from the tar-like pigment based ink residue in a jerking or stuttering type of motion, which fails to remove the residue from the printhead. In some cases, during this faster wiping stroke the wiper for the pigment based ink flexed and wiped over the tar-like residue, which smeared the ink over the orifice plate rather than removing it. Thus, any compromise in attempting to accommodate the wiping needs of one pen was at the sacrifice of meeting the needs of the other type of pen.
As the inkjet industry investigates new printhead designs, the tendency is toward using permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. Recent breakthroughs in technology have given hope to developing a printhead with a 25 mm swath height (about one inch high), which is double the height previously obtainable, and future developments may bring about even wider swath printheads. While there are a variety of advantages associated with these off-axis printing systems, the possibility of a wider swath height brings on other problems which have not previously been encountered, such as how to provide a uniformly adequate seal when capping the longer printhead, and how to seal the longer printhead without de-priming the nozzles. Moreover, the permanent or semi-permanent nature of the off-axis printheads requires special considerations for servicing, such as how to store ink spit over the printhead lifetime, and how to wipe ink residue from the printheads without any appreciable wear that could decrease printhead life.
To accomplish this wiping objective, an ink solvent, such as a polyethylene glycol ("PEG") compound, has been used in the HP HP 2000Ccolor inkjet printer, sold by the Hewlett-Packard Company. In this system the ink solvent is stored in a porous medium such as a plastic or foam block in intimate contact with a reservoir, with this porous block having an applicator portion exposed in such a way that the elastomeric wiper can contact the applicator. The wiper moves across the applicator to collect PEG, which is then wiped across the printhead to dissolve accumulated ink residue and to deposit a non-stick coating of PEG on the printhead face to retard further collection of ink residue. The wiper then moves across a rigid plastic scraper to remove dissolved ink residue and dirtied PEG from the wiper before beginning the next wiping stroke. The PEG fluid also acts as a lubricant, so the rubbing action of the wiper does not unnecessarily wear the printhead. Unfortunately, this solvent system uses many parts to accomplish this wiping routine, with multiple parts requiring multiple tooling costs, ordering, inventory tracking and assembly. Moreover, over the lifetime of the printer, the PEG ink solvent may need to be replenished to maintain optimum printhead servicing.